The invention relates generally to cardiac pacers, and more particularly to means for preventing crosstalk between bipolar pacer leads.
There are two major pumping chambers in the heart, the left and right ventricles. Simultaneously contracting, these chambers expel blood into the aorta and the pulmonary artery. Blood enters the ventricles from the left and right atria, respectively. The contractions arise from a wave of electrical excitation which begins in the right atrium and spreads to the left atrium. The excitation enters the atrio-ventricular (AV) node which delays its passage via the bundle of His into the ventricles. The atria contract in a separate action which precedes the major ventricular contraction by an interval of about 100 milliseconds (ms), known as the AV delay. Atrial contractions begin every 400-1,000 ms at a steady metabolically determined frequency known as the "sinus" rate, which increases with exercise, the AV delay being foreshortened at higher rates.
Electrical signals corresponding to the contractions appear in the electrocardiogram. A signal known as the P-wave accompanies atrial contraction while a signal known as the QRS complex, with a predominant R-wave, accompanies the ventricular contraction. The P and R-waves can be reliably detected as timing signals by electrical leads in contact with the respective heart chambers.
The typical implanted cardiac pacer operates by producing stimulation pulses to supply missing excitation via an insulated wire (or "pacing lead") terminating distally in an electrode attached to the right ventricle. The R-wave can be sensed by the same lead to inhibit or trigger stimulation or to restart a timing interval as in "demand" pacing. An additional lead contacts the atrium to sense P-waves, if desired. Pacers whose ventricular stimulation is timed from the sensing of a P-wave are referred to as AV synchronous or "physiological" pacers since they preserve the natural sinus rate as well as the normal sequence of contractions. In AV sequential pacers, sometimes the atrial lead is also used for atrial stimulation. Examples of physiological AV sequential pacers or "double demand" pacers in which the atrial and ventricular leads can both stimulate and sense are shown in pending U.S. Patent Application Ser. No. 153,422 entitled "Ventricular Inhibited Cardiac Pacer" filed May 27, 1980 and U.S. Patent Application Ser. No. 207,003 entitled "Multi-Mode Microprocessor Based Programmable Cardiac Pacer" filed Nov. 14, 1980, both assigned to the assignee of the present application, and incorporated herein by reference in their entirety.
There are two basic types of electrode systems used in pacing leads. Unipolar leads terminate distally in a single electrode (cathode) and employ the case of the pulse generator itself, or a conductive plate on the case, as the return electrode or ground (anode). Bipolar pacing leads, on the other hand, terminate distally in two spaced insulated electrodes connected to the pulse generator through respective wires in the pacing lead. Thus, each bipolar lead carries a positive and negative electrode for the respective chamber, and the case is not designed to form a part of the electrical circuit in this configuration.
In an AV sequential bipolar lead pacing system, bipolar pacing leads extend into the right atrium and right ventricle. In a pacer having a common ground connection, the two positive electrodes on the respective bipolar leads are tied together electrically. This shared ground connection can present crosstalk problems in both sensing and stimulation when each bipolar lead is in a different heart chamber. This is an extremely important problem to solve for physiological pacers which provide bipolar stimulation and sensing for both heart chambers with the same implanted pacer powered by a single battery.
One of the ways previously used to accomplish some measure of isolation between bipolar leads is to employ a transformer to couple the output stage of the pacing circuit to one of the leads to isolate the bipolar lead electrodes from each other. This approach, however, has only been practical when sensing is done only on one channel. In addition, it has the serious drawback of adding a relatively bulky inefficient component to the otherwise miniaturized pacer electronics.